Stability of major ions and sampling variability in daily bulk precipitation samples

Abstract

Daily (24-h) precipitation samples were collected in four parallel bulk samplers in order to examine differences between parallel samples and the maximum acceptable time delay between sampling and analysis. After an immediate analysis, the samples were stored at +4°C over a seven-week time period to regularly examine the changes in ionic composition. The experiment was carried out in Helsinki, Finland, over the period from June 1997 to June 1998, with six rain events collected. Electrical conductivity, pH and the concentrations of the major ions Na +, NH 4 +, K +, Mg 2+, Ca 2+, Cl −, NO 3 − and SO 4 2− were studied. No significant differences between the parallel samplers were observed showing that dry deposition does not usually result in increased variability for 24-h bulk sampling due to the short dry period exposed. Sodium, potassium, magnesium, calcium, chloride, nitrate and sulphate concentrations remained constant for at least seven weeks. The electrical conductivity decreased by up to 30% and the pH increased by up to 0.3 pH unit, in thunderstorm sample even 1 pH unit, after six days. The ammonium concentration had a slight decreasing trend in all rain events and in some events the concentration decreased by upto 25% during the seven weeks storage. To achieve as precise results as possible for pH, electrical conductivity and ammonium the recommended maximum time delay between sampling and analysis is one week.